1. Field of the Invention
The present invention relates to snubber circuit configurations for protecting semiconductor elements during operation of a power conversion assembly including one or more self turn off power semiconductor elements.
2. Description of Related Art
It is well known that by use of a pair of power transistors, for example, connected in series as a self turn off power semiconductor element, with this series circuit connected between the positive and negative terminals of a DC power supply, DC to AC power conversion can be achieved by alternating between the ON and OFF states of the two power transistors. For three phase AC output, three pairs of series circuits of such power transistor pairs are connected in parallel, commonly connected to a DC power supply. Hereafter, the prior art and the present invention will be explained in detail with examples of how DC power from a DC power supply is converted to 3-phase AC power by a 3-phase inverter by the use of three similar assemblies.
The ON to OFF condition of each of the six transistors typically part of a three phase DC to AC inverter is determined by the driving circuits not shown herein, but well known in the art. It is the function of the drive circuits associated with such an inverter as described herein to insure that the transistors will go into saturation when turned ON and will be cut off, or turned OFF at the proper time. The drive circuits will supply the transistors a base drive signal so that when one transistor of a series string is ON, the other is OFF. Furthermore, the relationship between the ON to OFF time of the three strings of two series transistors each will be arranged so that the relative timing relationships of the three phases of an AC power line, usually desired to be spaced 120 electrical degrees apart, will be preserved.
The ON to OFF time ratio of the series pair of transistors in each of the three two transistor strings is determined by the load that the inverter is supplying. This ratio is determined by the control circuitry associated with the drive circuits not shown herein.
Note that the two transistors can never be ON at the same time, or else they would appear as a short across the DC supply, and self-destruct. Therefore, it is imperative that high energy spikes created during the switching of the power devices be well attenuated in the snubber circuits to allow the unimpeded operation of the transistor drive circuits.
In a 3-phase inverter formed by power transistors, each power transistor is typically connected reversely in parallel with a free wheeling diode. In this configuration when a surge voltage, induced by (di/dt) is generated by the stray inductance of the circuit excited by the power transistor being turned OFF (or the reverse recovery time of the free wheeling diode), the power transistor in question may be damaged. Moreover, when the rising rate (dV/dt) of the collector to emitter voltage of such power transistor is large while the power transistor turns OFF, the other power transistors or control circuits may malfunction. Accordingly, a snubber circuit in accordance with this invention is incorporated in a 3-phase inverter in order to avoid such malfunctions, and generally reduce the dV/dt effects.
FIG. 19 is a circuit diagram indicating a first example of a prior art snubber circuit incorporated in a power conversion unit.
In FIG. 19, the numerals 1 to 6 designate six power transistors and 7 to 12, the associated six free wheeling diodes. Each of these six power transistors is connected "in parallel" in the form of a 3-phase bridge as shown with the free wheeling diodes and each power transistor also having connected in parallel with them a snubber circuit formed by a series connected capacitor and resistor. The numerals 13 to 18 designate six capacitors forming part of such snubber circuit. Numerals 33 to 38 designate resistors also part of the snubber circuit. The numeral 25 designates a DC power supply and 32, the stray inductance of the circuit.
Such snubber circuit formed by resistors and capacitors is generally called an R-C snubber circuit. The capacitors comprised within such R-C snubber circuit suppress a surge voltage generated when the power transistor connected to this R-C snubber circuit turns OFF. The circuit also suppresses high dV/dt of the collector to emitter voltage of this power transistor through charging of the these snubber capacitors. It is preferable to minimize the resistance value of the resistors forming the R-C snubber circuit in order to improve the suppression effect of the possibly destructive high dV/dt surge condition.
FIG. 20 is a circuit diagram indicating a second example of a prior art snubber circuit incorporated in a power conversion unit.
In FIG. 20, six power transistors 1 to 6, six associated free wheeling diodes 7 to 12, a DC power supply 25 and a stray inductance 32 represent the same function as those in FIG. 19.
In the second prior art circuit shown in FIG. 20, the snubber circuit (generally called a R-C-D snubber circuit) formed by the resistor 28, capacitor 30 and diode 27 is connected between the positive and negative electrodes of the DC power supply 25. In this configuration, the capacitor 30 absorbs the energy induced by the stray inductance 32 of the circuit when the power transistor turns OFF, thereby suppressing the surge voltage, or high dV/dt condition.
FIG. 21 is a circuit diagram showing a third example of a prior art snubber circuit incorporated into a power conversion assembly.
FIG. 21 combines the first prior art circuit shown in FIG. 19 and the second prior art circuit shown in FIG. 20 and described previously. The six transistors forming a 3-phase DC to AC inverter are connected respectively in parallel with an individual R-C snubber circuit and also connected with the R-C-D snubber circuit across the positive and negative electrodes (collector and emitter) of the six transistors.
FIG. 22 is a circuit diagram indicating a fourth prior art form of snubber circuit incorporated in a power conversion unit. The six transistors forming a 3-phase inverter are connected in parallel with individual R-C-D snubber circuits and are also connected with the R-C-D snubber circuit across the positive and negative electrodes of the power transistors.
In the prior art circuits described above, if the diode or resistor connected in series with the capacitor forming the snubber circuit is removed to form a snubber circuit consisting of the capacitor only, when the power transistor turns OFF, a resonant phenomenon is generated by the capacitor and stray inductance 32 of the circuit. Since this phenomenon creates problems such as the generation of noise and an increase in the duty cycle of the power transistor, a resistor is connected in series with the capacitor to suppress the electrical waveforms generated by the resonance.
The R-C snubber circuit and R-C-D snubber circuit of the prior art described above have various disadvantages.
In the R-C-D snubber circuit, 1) the resistor must be made large in size because of the losses associated with the current required to charge the capacitor and also discharged through such resistor; 2) because of the presence of the resistor, a control circuit may be improperly triggered by a malfunction caused by the high dV/dt spike generated when the diode of the snubber circuit recovers reversely; and 3) the part count is necessarily high because the snubber circuit is relatively complicated in structure.
Meanwhile, in the plain R-C snubber circuit, 1) the resistor energy loss becomes large since the capacitor is charged or discharged through the resistor and 2) because of the associated power dissipation, the resistor is large in size.
In addition, these snubber circuits further provide disadvantages in that each connection of elements forming the snubber circuits to respective semiconductor elements for power conversion not only results in troublesome assembly procedures but also increases the wiring inductance between the semiconductor element for power conversion and the snubber circuit. This also results in a part count increase of the snubber circuits and a corresponding increase in the size of the conversion unit or assembly.
It is therefore an object of the present invention to realize reduction in parts count and energy loss through a simplified structure of the snubber circuit incorporated in the semiconductor assembly for power conversion. Also, reduction in size of the power conversion unit using snubber circuits and reduction of wiring inductance through integration of the semiconductor element and the snubber circuit is also a goal.